Anaerobic digestion is a common unit operation employed in the treatment of wastewaters containing organics and nitrogenous compounds including industrial, municipal and agricultural wastewaters. The resulting solid/liquid slurry from an anaerobic digester has a high-solids portion and a low-solids portion. For example, the digestate produced from dairy wastewater, contains a high-solids portion comprising largely cellulosic solids and a low-solids portion containing concentrations of dissolved carbon dioxide and dissolved ammonia nitrogen as well as salts and both suspended and dissolved organics. The dissolved ammonia nitrogen in the digestate presents significant environmental issues if left untreated, such as, when the digestate is land applied, discharged to a body of water, or sent to a holding pond or lagoon. Potential adverse air and water impacts include: ammonia (toxic to fish, irritating to human eyes and lungs) will be lost both to the air and water; ammonia will be biologically oxidized either in water or soil and chemically oxidized in the air, in either case forming gases that are irritants and can form ozone or greenhouse gases, etc. Most often regulations for ammonia release are designed to prevent excess nutrient input to surface waters which may cause eutrophication.
Effective treatment technologies are needed for agricultural and industrial waste streams that may release ammonia to the environment. For example, anaerobic digester digestate is often high in ammonia and their sources are required to remove ammonia nitrogen to avoid excessive nitrogen discharges.
One well-established technology for treatment of digestate is air stripping which uses hot air and/or steam to strip ammonia from the wastewater creating a liquid stream comprising substantially less dissolved ammonia and a heated gas containing the stripped ammonia. Formation of a solid precipitate containing ammonia, ammonium bicarbonate, and ammonium carbonate, during the air stripping process may foul the air stripping substrates causing operational and maintenance issues and thus is not desired. An increase of the pH of the wastewater shifts the equilibrium for ammonia away from dissolved ionized ammonium and more to ammonia gas. Accordingly, increased removal of the ammonia from digestate using air stripping is commonly achieved with chemical addition. Examples of chemicals used to increase pH include calcium, sodium or magnesium hydroxide. The stripped ammonia is absorbed into an acid solution. The use of acids is highly effective for ammonia recovery and could also be effective in producing a concentrated ammonium salt product using subsequent unit operations. On the other hand, treatment utilizing industrial chemicals to raise pH with a stripping process and for absorption of ammonia, such as, for example, for a treatment system for a dairy waste which produces a nutrient rich solid for use as a fertilizer, has the unavoidable consequence stemming from the use of such chemicals that any reusable end product cannot be certified as an “organic” product.
Stripper exhaust gas containing ammonia is sometimes released to the atmosphere although regulations typically require that it is further processed to capture the nitrogen. For example, U.S. Pat. No. 7,811,455 (Burke) describes a process for use of biogas rather than air and reclaiming ammonia from stripper exhaust gases in the form of ammonium bicarbonate by blending the CO2 in the digester biogas with the stripper gas and then precipitating and recovering ammonium bicarbonate with the added benefit of lowering CO2 in the biogas. One of the main drawbacks with that process is that the use of chemicals to raise the pH in the stripper precludes certification of the ammonium bicarbonate solids and any solids created from the ammonium bicarbonate solids as “organic” fertilizer. Another drawback is the inefficiency associated with processing large volumes of gas, including precipitation of ammonium bicarbonate in the gas phase.
There is a need for a waste treatment technology that converts wastewater containing nutrients into beneficial use materials that can be certified as organic. There is a need for a wastewater treatment technology that can remove dissolved ammonia nitrogen, react the resulting gaseous ammonia with carbon dioxide to re-form dissolved ammonium bicarbonate, concentrate the dissolved ammonium bicarbonate, and then capture the ammonium bicarbonate in crystalline form. There is a need for an improved manure treatment system comprising anaerobic digestion that does not utilize expensive, hazardous, chemicals to raise the pH of digestate, chemicals that present significant handling and storage issues. There is a need for an improved wastewater treatment system that effectively removes and recovers nitrogen in the form of ammonium bicarbonate without any chemical addition.